1. Field of the Invention
The present invention relates generally to evaporative emission control in motor vehicles. More particularly, the invention relates to a fuel weathering method for a vehicle evaporative emission system.
2. Background Art
Conventional motor vehicles are well known to release evaporative hydrocarbons into the atmosphere during both operating and non-operating states of the vehicle. Consequently, laws and regulations have been established requiring on-board vehicle evaporative emission systems to control the amount of fuel vapors emitted into the atmosphere. Such systems typically include a carbon filled canister and one or more valves for collecting, routing and venting unburned hydrocarbon emissions.
To monitor the level of hydrocarbon emissions from such systems, so-called On-Board Diagnostics (OBD) systems are used to insure that a vehicle""s evaporative emission system and powertrain components are operating in compliance with government standards. Conventional diagnostic systems, including OBD systems, utilize pressure or vacuum tests to monitor hydrocarbon emissions. Generally, these systems apply a partial vacuum to the fuel tank of the vehicle until a predetermined pressure level is reached. Once the predetermined pressure level is reached, the tank is sealed and the system measures the amount of vacuum xe2x80x9cbleed offxe2x80x9d over a predetermined period of time. An example of one such diagnostic system is described in U.S. Pat. No. 5,261,379 to Lipinski et al., which is also owned by the assignee of the present application.
Conventional diagnostic systems, however, do not take in account evaporative characteristics of the liquid fuel inside the vehicle""s evaporative emission system, and as such detection of low-level hydrocarbon emissions is limited under most conditions. This occurs because excessive evaporation of certain fuel constituents, for example, can cause an erroneous indication of vacuum xe2x80x9cbleed offxe2x80x9d. Accordingly, the inventors herein have recognized the need to provide a xe2x80x9cfuel weatheringxe2x80x9d method that takes into account the evaporative characteristics of fuel inside a vehicle evaporative emission system.
The aforedescribed limitations and inadequacies of conventional evaporative emissions diagnostic systems are substantially overcome by the present invention, in which a xe2x80x9cfuel weatheringxe2x80x9d method is provided having the steps of: determining a current temperature of liquid fuel inside a vehicle evaporative emission system; determining a fuel residency time representing the amount of time the fuel has resided above the current temperature; and performing a systems diagnostic test of the evaporative emission system only if the fuel residency time exceeds a minimum fuel residency time. By tracking the amount of time the fuel has resided above the current temperature, the fuel weathering method of the present invention accounts for fuel evaporative effects over time and thus improves the emissions monitoring capabilities of the vehicle.
A corresponding system is also disclosed having a sensor for measuring a current temperature of liquid fuel inside the evaporative emission system, and a controller coupled to the sensor for determining a fuel residency time representing the amount of time the fuel has resided above the current temperature, and for performing a system diagnostic test of the evaporative emission system only if the fuel residency time exceeds a minimum fuel residency time.
An advantage of the above-described fuel weathering method and corresponding system is that the accuracy of evaporative emission system diagnostic testing is improved by minimizing test-to-test variability within the same tank of fuel. In accordance with the fuel weathering method disclosed herein, the controller keeps track of how much time the fuel liquid temperature has spent above various temperatures since the last refueling event. The controller then allows the diagnostic test to proceed only if the fuel has spent enough time above the current test temperature. This ensures the fuel has been sufficiently xe2x80x9cweatheredxe2x80x9d by insuring that the diagnostic test is not performed until certain constituents of the fuel at the current temperature have already been vaporized at higher temperatures and have been removed from the fuel by the evaporative emission system.